A two-way voice communication instrument, such as a telephone handset or headset, must be interfaced to the telephone line communication link. Communication interface apparatus typically includes amplifier circuitry to provide voice signal conditioning. However, there are variations in the voice communication characteristics of the instruments, which require different signal conditioning by the interface apparatus. Heretofore, different interface apparatus has been required in order to provide appropriate signal conditioning for a particular voice communication instrument. The present invention provides a single interface apparatus that is compatible with a broad range of voice communication instruments.